Navigate to https://www.netflix.com and logon as needed (will be needed the first time for a new profile). When redirected to https://www.netflix.com/browse the page will freeze, the CPU will hit 25% or slightly more (equivalent of saturating a core of this 4-core CPU), and will stay there between 32 and 53 seconds. Then the page will become responsive. Do a page refresh, and the page will be frozen for another 32 to 53 seconds.

If I open another tab, and key in about:performance, the performance page shows that when https://www.netflix.com/browse refreshes that the Adblock Plus is using 61% of the CPU and the about:performance tab is using 20%. (which is odd; 61% + 20% = the 35% that Windows Task Manager is showing me?) Then after whatever it is that is grabbing the CPU cycles and the page has finished refreshing, Adblock Plus settles down to 1% of the CPU.

For what it's worth, when using Chrome 58.0.3029.110 (64-bit) with AdBlock Plus the CPU is pegged a bit north of 25% for only about 5 seconds, and the Chrome Task Manager shows AdBlock Plus peaking at 2.7% (when AdBlock Plus is set to "disable on netflix.com") and the tab for Netflix peaking at about 32%. These numbers are probably reasonable, considering the amount of junk https://www.netflix.com/browse drags in to produce all those customized suggestions.

PS: the about:performance page should be taken with a grain of salt. I also have a bug at Bugzilla about the about:performance page. When the rendering process is saturating the CPU, it becomes unresponsive to the parent process, so the tab using that process disappears from the about:performance page.

I have the same problem for about a week now. Last stable FF. Made sure I enabled multiprocess in Firefox by disabling incompatible addons. Netflix and its related domains are whitelisted in ABP, but it doesn't help. All works fine in Chrome with ABP. Any links for a Firefox bug where they say it's 100% ABP related? For me the site is very slow in safe mode too.

For completeness (for those who find this thread from doing a search), at least for the (streaming side of) Netflix slowdown, I'll reference this issue so others know that it is something the developers are aware of:

The bugfix helped with all other sites, but netflix.com is still mostly unusable (with last stable FF, multiprocess). Same as before: scrolling is slow, the tab affects other tabs, sometimes the message about slowdown appears.

Although it's better then before turning off "Count hits" in 2.9.1, browsing any page on netflix.com is very, very slow and most times blocks the whole browser causing the "slow" message to appear.I have to use Chrome just to be able to browse Netflix! And all for about 2 months now.Reference: viewtopic.php?f=1&t=52577